The first signal is evoked by the T-cell receptor after engagement with the major histocompatibililty complex-peptide complex expressed on the surface of antigen-presenting cells (APCs).

• 当T细胞受体(T cell receptor TCR)与存在于抗原呈递细胞(antigen presenting cell APC)表面的主要组织相容性复合物(major histocompatibility complex MHC)相结合后，第一信号被激活；